The present invention generally relates to fabrication methods and resulting structures for semiconductor/magnetic devices, and more specifically, to reducing shorts in magnetic tunnel junctions (MTJs).
Magnetoresistive random access memory (MRAM) is a non-volatile memory that combines a magnetic device with standard silicon-based microelectronics to achieve the combined attributes of non-volatility, high-speed read/write operations, high read/write endurance, and data retention. The term “magnetoresistance” describes the effect whereby a change to certain magnetic states of the MTJ storage element (or “bit”) results in a change to the MTJ resistance, hence the name “Magnetoresistive” RAM. Data is stored in MRAM as magnetic states or characteristics (e.g., magnetization direction, magnetic polarity, magnetic moment, etc.) instead of electric charges. In a typical configuration, each MRAM cell includes a transistor, a magnetic tunnel junction (MTJ) device for data storage, a bit line, and a word line. In general, the MTJ's electrical resistance will be high or low based on the relative magnetic states of certain MTJ layers. Data is written to the MTJ by applying certain magnetic fields or charge currents to switch the magnetic states of certain MTJ layers. Data is read by detecting the resistance of the MTJ. Using a magnetic state/characteristic for storage has two main benefits. First, unlike electric charge, magnetic state does not leak away with time, so the stored data remains even when system power is turned off. Second, switching magnetic states has no known wear-out mechanism.
Spin-transfer torque (STT) is a phenomenon that can be leveraged in MTJ-based storage elements to assist in switching the storage element from one storage state (e.g., “0” or “1”) to another storage state (e.g., “1” or “0”). For example, STT-MRAM uses electrons that have been spin-polarized to switch the magnetic state (i.e., the magnetization direction) of a free layer of MTJ. The MTJ is configured to include a reference/fixed magnetic layer, a thin dielectric tunnel barrier, and a free magnetic layer. The MTJ has a low resistance when the magnetization direction of its free layer is parallel to the magnetization direction of its fixed layer. Conversely, the MTJ has a high resistance when its free layer has a magnetization direction that is oriented anti-parallel to the magnetization direction of its fixed layer.